Cement for spark plugs and the like



CROSS REFERENCE A. V. SOMERS ETAL CEMENT FOR SPARK PLUGS AND THE LIKE Sept. 29, 1959 Filed Sept. 26, 1957 United States Patent CEMENT FOR SPARK PLUGS AND THE LIKE Arthur V. Somers, Flushing, and Karl Schwartzwalder, Holly, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application September 26, 1957, Serial No. 686,410

9 Claims. (Cl. 313-145) This invention relates to spark plugs and to an improved sealing cement having particular utility for securing and sealing the metal centerwireelements into the centerbore of a ceramic spark plug insulator. An object of the invention is to providem'ed heat resistant sealing cement having particular utility in spark plugs and the like. Another object of the invention is a spark plug having an improved bond and seal between one or more of the metal centerwire elements and the ceramic insulator.

In the drawing there is shown a side view in section of a representative spark plug having a metal shell 2 with a tubular ceramic insulator 4 concentrically enclosed therewithin. In the insulator centerbore is the centerwire assembly 6 including as elements a center electrode 8, a conductive glass seal 10, metal contact 12 which is embedded in the glass seal, a cartridge resistor element 14, contact spring 18 and a threaded contact cap 20. The threaded contact cap 20 is sealed within the insulator centerbore by means of cement 22 which is the subject of the present invention. Thus, in the particular spark plug shown, which is of the resistor type and designed for aircraft use, the cement is used to anchor and prevent loosening by vibration of the resistor contact cap and also to prevent extraneous materials from entering the resistor well which is formedby the center portion of the insulator centerbore. In other types of spark plugs, the cement may be used to seal and secure other centerwire elements within the centerbore, i.e., other contact elements, the resistor cartridge, the electrode spindle or the like.

In accordance with the invention, the spark plug centerwire element is secured and sealed within the insulator centerbore with a cement made by mixing about 3 parts by weight cglgumal umifnatbe with from 2 to 3, preferably from 2 to 2.5, par s y weight of an aqueous sodium silicate sglution twaterglass) as will be hereinafter escnbe with more particu ars.

As raw material for the cement, solid technical grade calcium aluminate, which may contain calcium oxide as an impurity, can be used. Generally the calcium aluminate, being hygroscopic, will also contain a small amount of water. The aqueous sodium silicate should contain from 50% to 65% water and have a Na O-to-SiO ratio from 1:15 to 1:2.5. Cements otherwise the same, compositionwise, as those comprehended by the present invention but made with proportions of calcium aluminate and aqueous sodium silicate different than those specified, or made with aqueous sodium silicates containing proportionate amounts of water, Na O and SiO, outside the above-specified ranges have been found to be unsatisfactory, particularly in that they are grainy, or harden too rapidly, or tend to boil on curing. By far the preferred aqueous sodium silicate for the practice of the invention is one containing approximately 55% water and a Na O- to-SiO ratio of about 1:2. A typical analysis of the preferred sodium silicate is: Na O14.7% SiO,-29.4%

mainder water) 420 Calcium aluminate 600 Sodium silicate (about 15% Na O, 30% SiO remainder water) 432 Water (added as such) 48 To make the cement, the calcium aluminate, usually lumpy as purchased, is broken up an m1 e to reduce it to a relatively uniform fine grain size. The milling can be accomplished in about one hour in an ordinary ball mill. A weighed amount of the milled calcium aluminate is placed in a mixer and the proper proportionate amount of sodi icate having a composition as specified above i a ded r y with constant mixing. If additional water is desired to reduce the viscosity of the cement, it can be added with further mixing after the addition of sodium silicate is completed. Agitation is continued until a uniform mixture is obtained and the cement can then be used immediately or stored, preferably in lead tubes. It has been found that the cement can be stored for a longer period of time if it is maintained at about 40 F. or less, preferably at less than 10 F.

To manufacture spark plugs of the type shown in the accompanying drawing, the cement is used in the following manner. A small portion of the cement is squeezed from the lead storage tube onto the threads of the contact cap 20, after which the cap is screwed into the insulator. Then the structure is W hours at about and subsequently ba e in an oven at for from 30 minutes to two hours to thoroughly dry and set the cement. The most preferable drying and baking temperatures and times will, of course, depend uponjmredsTg ggposjtmaiJbb cement. In general, the less e water content of the a I I cement, the less the tlmes and temperatures required for drying and baking.

Cements made in accordance with the present invention have very hgwmw and strength and are I simple to manu ac ure. They possess a relatively long useful application-processing life in that they can be kept at room temperature for up to five hours without hardening. With prior cements used for the same purpose, application had to be accomplished in a very short period lest there be premature hardening. The cement also facilitates processing since, unlike many previous cements, it does not tend to boil out during curing and eliminates the necessity for lengthy drying periods.

If desired, inert filler materials such as fibrous mullite,

lass, asbestos, alumina, zircon or the like may be mixed with the cement to provide additional strength and adjust the consistency to that desired and the appended claims are intended to comprehend such additions, as well as additions of water or other diluents to the cement.

We claim:

1. A cement consisting essentially of the product resulting from mixing 3 parts by weight calcium aluminate with from 2 to 3 parts by weight of an aqueous sodium silicate solution containing about 50% to 65% by weight Patented Sept. 29, 1959 3 r water and having a Na O-to-SiO ratio of from 1:1.5 to 1:2.5.

2. A cement consisting essentially of the product resulting from mixing 3 parts by weight calcium aluminate with about 2 to 2.5 parts by weight of an aqueous sodium silicate solution containing from 50% to 65% by weight water and having a Na O-to-siO ratio of from 1:1.5 to 1:2.5.

3. A cement consisting essentially of the product resulting from mixing about 3 parts by weight calcium aluminate with from 2 to 3 parts by weight of an aqueous sodium silicate solution containing approximately 55% by weight water and having a Na O-tQ-SiO, ratio of about 1:2.

4. A cement for spark plugs and the like consisting essentially of the product resulting from mixing about 3 parts by weight calcium aluminate with from 2 to 2.5 parts by weight of an aqueous sodium silicate solution containing approximately 55 by weight water and having a Na O-to-sio ratio of about 1:2.

5. A cement for spark plugs and the like consisting essentially of the product resulting from mixing about 6 parts by weight calcium aluminate with about 4.2 parts by weight of an aqueous sodium silicate solution containing approximately 55 by weight water and a Na O-to- SiO, ratio of about 1:2.

6. A cement for spark plugs and the like consisting essentially of the product resulting from mixing about 6 parts by weight calcium aluminate with about 4.32 parts by weight of an aqueous sodium silicate solution con- 4 taining approximately 55% by weight water and a Na oto-SiO ratio of about 1:2.

7. A method for making cement comprising the steps of mixing from 2 to 3 parts by weight of an aqueous sodium silicate solution containing to 65% by weight water and a Na O-to-SiO, ratio of from 1:15 to 1:2.5. with 3 parts by weight calcium aluminate, and then adding water to adjust the viscosity of the cement.

8. A spark plug comprising a ceramic insulator having a centerbore therethrough and a metal element secured within said centerbore by a cement, said cement being the product resulting from drying and then baking a mixture consisting essentially of 3 parts by weight calcium aluminate and from 2 to 2.5 parts by weight of an aqueous sodium silicate solution containing from 50% to 65% by weight water and having a Na O-to-SiO ratio of from 1:1.5 to 1:2.5.

9. A spark plug comprising a ceramic insulator having a centerbore therethrough and a metal element secured within said centerbore by a cement, said cement being the product resulting from drying and then baking a mixture consisting essentially of 3 parts by weight calcium aluminate and from 2 to 2.5 parts by weight of an aqueous sodium silicate solution containing approximately by weight water and having a Na o-to-sio ratio of about 1:2.

References Cited in the file of this patent UNITED STATES PATENTS 

8. A SPARK PLUG COMPRISING A CERAMIC INSULATOR HAVING A CENTERBORE THERETHROUGH AND A METAL ELEMENT SECURED WITHIN SAID CENTERBORE BY A CEMENT, SAID CEMENT BEING THE PRODUCT RESULTING FROM DRYING AND THEN BAKING A MIXTURE CONSISTING ESSENTIALLY OF 3 PARTS BY WEIGHT CALCIUM ALUMINTE AND FROM 2 TO 2.5 PARTS BY WEIGHT OF AN AQUEOUS SODIUM SILICATE SOLUTION CONTAINING FROM 50% TO 65% BY WEIGHT WATER AND HAVING NA2O-TO-SIO2 RATIO OF FROM 1:15 TO 1:25. 